(7d) Exploring Fundamentals of Zeolite Catalysis – a Theoretical Perspective

Zeolites are among the most important industrial catalysts today. Their diverse structure leads to a large variety of different possible active sites combined with a high selectivity due to their microporous structure. However, chemical reactions take place inside the material, which makes a full understanding of them highly challenging. This is one of the reasons, why many fundamental aspects of zeolite catalysis are still little understood. In this contribution I describe my efforts in the past years to elucidate several of the underlying principles in context of several timely problems within zeolite catalysis.

One of the main challenges in automotive catalysis using lean burn engines is the removal of nitrous oxides from exhaust gases. A system that has been found to be a particularly efficient catalyst in this context is copper exchanged SSZ-13 (Cu-SSZ-13). Even though SSZ-13 is a zeolite in the chabazite structure, the zeolite structure with the most simple primitive unit cell, the true nature of the active centers for this reaction is still under discussion. In this contribution I describe my efforts to combine ab-initio molecular dynamics simulations, post Hartree-Fock calculations and thermodynamic modeling to arrive at an unambiguous assignment of infrared spectra of adsorbed probe molecules [1,2] and UV-vis spectra of reduced Cu-centers [3]. Due to the thermal motion at finite temperature, the Cu cation is highly mobile within the framework. This mobility changes with the oxidation state. While Cu^II oscillates around its local minimum, Cu^I changes its coordination on a pico-second timescale [1,2,4] and Cu⁰ even moves from its favored position in the six-ring of the structure to silanol-defect sites [3]. However, as soon as water is present in the system, the cation is solvated and moves away from the framework. While it is possible to follow this mobility by comparing calculated spectra to experimentally measured ones, it is still exciting to see in what way these fundamental insights on the nature of the active sites will affect our understanding of the reaction cycle and whether they can be transferred to other zeolite catalyzed reactions.

Another reaction that has recently drawn large amounts of attention is the conversion of methane-to-methanol over transition metal exchanged zeolites. First of all these systems show a very low activation energy and at the same time the active centers are very similar to those in enzymatic catalysts, responsible for catalyzing this reaction in nature. Here I will discuss two different aspects of this problem. It is safe to assume that the confining environment is a key factor for the activity of these materials. Confinement effects are rather complicated to understand, since entropy and enthalpy affect adsorption and conversion of reactants and products. A first step is understanding the impact of confinement on the activation enthalpy of the methane-to-methanol conversion over Fe-oxo centers in SSZ-13 [5]. I conclusively show that confinement stabilizes the reaction intermediates more than the initially adsorbed methane molecule, which in turn leads to a significant lowering of the activation energies. This allows to extrapolate generally valid statements about the impact of confinement on reaction enthalpies, which will most likely impact our understanding of similar processes in the future. However, more recently Cu-exchanged zeolites have drawn larger amounts of attention in the scientific community for this reaction. I will present a thermodynamic model for Cu_xO_yH_z sites in the zeolite SSZ-13 [6]. Interestingly not only one, but several different types of active sites exist in this material and depending on the exact conditions, different ones are thermodynamically most stable. The theoretical model is also supported by Raman- and UV-vis measurements, which show excellent agreement with the modeled results.

The problems discussed in this contribution show, how of state of the art electronic structure theory combined with experimental measurements can help elucidating fundamental concepts in zeolite catalysis. In the future I am looking forward to continue my work along this path to improve the conceptional understanding of zeolite catalysis, to generalize these ideas and apply them to catalysts of industrial interest and ideally arrive at an approach that does not only help to understand, but also predict the activity of a large number of zeolite catalysts. Furthermore I belive that collaboration is one of the key aspects of succesful scientific work and I am looking forward to establish joint projects with other faculty members.

Teaching Interests:

Besides my achievements in science, I have always had a passion for learning and teaching, which has been reflected in mentoring graduate and undergraduate students as well as teaching selected classes during my current post-doc. In my time as a faculty member I plan to develop a course describing how state of the art electronic structure methods can be applied to gain insights into heterogeneous catalysis, either by understanding reaction pathways or by predicting observed spectra. It will provide the necessary theoretical background as well as practical examples and hands-on experience for the students. Among the undergraduate core classes I ready to teach thermodynamics and, depending on the departmental needs, am looking forward to develop classes close to my core competencies in fundamental physics, quantum mechanics, catalysis or materials science.